Currently, all high average power lasers (e.g., >>100 kW average power) with high beam quality (e.g., M2≈1) utilize gaseous laser gain media (e.g., CO2, CO, HF, DF, and O2—I). Although technologically well developed, their widespread deployment has been and is expected to be quite limited. In many high energy laser (HEL) applications deployment is limited because their mid-infrared output wavelengths (e.g., CO2, CO, HF, DF) are considered to be too long for intended applications, requiring excessively large-aperture transmitter optics that are both bulky and expensive. In addition, those sources powered by chemical reactions (e.g., HF, DF, O2—I) pose unacceptable operational safety risks and logistical field supply chain difficulties. Furthermore, the power gain media of these chemically based sources are incapable of storing excitation energy over long (e.g., multiple millisecond) durations. This precludes short duration (e.g., less than 100 nanoseconds), high energy output pulse generation needed in some high power laser applications.
Another type of laser was explored two decades ago to achieve high average power with short duration, high peak power pulses based on the use of the diatomic molecule sulfur monoxide. In this SO laser, a 193 nm argon fluoride (ArF) pulsed discharge laser was used to photodissociate sulfur dioxide (SO2) molecules to produce vibrationally excited, ground level SO molecules and atomic oxygen atoms. Then a 248 nm krypton fluoride discharge laser was used to resonantly excite a high lying (UV) SO electronic state. Laser action takes place between the pumped UV electronic state and a rotational-vibrational manifold of the ground electronic state. Because the efficiencies of the ArF and KrF discharge lasers are so low (a few percent), and because the conversion of KrF pump energy to SO laser output energy is so low (<<1%), the overall efficiency of this type of SO laser is impractically low and has remained a scientific curiosity. Furthermore, the radiative lifetime (i.e. maximum energy storage lifetime) of the high lying (UV) SO electronic state is only 35 nanoseconds.
A number of advanced laser technologies have been evaluated during the past two decades in various attempts to overcome the limitations cited above. These include: 1) high power semiconductor laser diodes, 2) Diode Pumped Solid State lasers (DPSSLs), and 3) Diode Pumped Alkali Lasers (DPALs).
A high power semiconductor laser diode, or an array of such diodes, is a powerful, efficient and compact laser source. However, such a laser diode or array source emits its output radiation in a beam that is many times greater than the diffraction limit of the emitting aperture. Thus, the radiation from such a semiconductor laser diode source cannot be propagated to a small spot at a large distance. Moreover, the semiconductor gain medium can only store excitation energy for a few nanoseconds. This generally precludes semiconductor laser diodes from generating high energy output pulses directly. For at least these reasons, semiconductor laser diodes, employed as direct sources of laser radiation, fail to overcome the beam quality and high peak energy pulse generation limitations of high power, high energy chemical lasers.
Notwithstanding these limitations, high power semiconductor laser diode and diode arrays can be used to optically excite the energy levels of a separate gain medium that may be capable of: 1) generating a high power CW output laser beam that is nearly diffraction-limited (often described as a “spatial mode converter”), and 2) storing the applied pump energy for long time durations suitable for generating high energy laser output pulses.
The semiconductor laser diodes containing Al, Ga, In, and As emit settable, relatively narrowband radiation in the 730-1100 spectral region. Semiconductor laser diodes can be designed to emit at wavelengths matched to well known absorption transitions of solid state laser materials, such as neodymium doped crystals and glasses, and ytterbium doped crystals and glasses. Thus, semiconductor laser diodes and diode arrays can be used to efficiently pump (excite) a rare earth doped solid state laser medium, producing a population inversion between certain levels therein. When the excited gain medium is contained within an appropriately designed laser resonator, a high fraction of the excitation energy may be extracted from the solid state gain medium. Because rare earth doped solid state gain media can store excitation energy over long durations (e.g., hundreds of microseconds to milliseconds), high energy output laser pulses may be realized in DPSSLs.
However, waste heat generated in a DPSSL due to internal loss processes must be transported to a heat sink positioned away from the laser gain region so as to not overly distort the output laser beam. In a solid state gain medium this heat transport must be achieved through the process of thermal conduction in the gain medium itself. This inevitably results in thermally-induced index-of-refraction gradients transverse to the laser resonator axis. These gradients deteriorate output beam quality, especially under high average power operation. It has proven difficult to mitigate this mechanism of output beam deterioration. Thus, DPSSLs with high beam quality remain power limited.
The DPAL was developed to exploit the power, compactness, and efficiency of semiconductor laser diodes while overcoming the thermally-induced beam quality distortion limitations of DPSSLs by utilizing atomic alkali atoms in the vapor phase as gain media. DPALs are electrically pumped and emit at shorter wavelengths than any of the aforementioned chemical gas and DPSSL lasers. However, a vapor gain medium introduces significant operational limitations related to controlling conditions at a liquid or solid alkali reservoir to generate the desired vapor conditions during operation. Moreover, the upper laser levels of the alkali atoms employed in DPALs possess relatively short energy storage lifetimes (e.g., approximately 30 nanoseconds). Thus, DPALs generally cannot be designed to produce the short duration, high energy output pulses required by a number of high energy, high power applications.
Thus, there is a continuing need for a non-chemical, electrically-powered, short-wavelength laser using a purely gaseous (i.e., non-vapor) gain medium that may be scaled to multi-kilowatt and above power levels with near diffraction-limited beam quality. Furthermore, it is desirable that the gaseous gain medium be able to store optical pump energy for a relatively long time (e.g., multiple milliseconds) to enable production of high energy, high-peak-power pulses on a repetitive basis.